Thermal History Of Sample Research Articles

Origin of the Ca-phosphate inclusions in Ivory Coast and Australasian Muong-Nong-type tektites

Tektites are reduced (Fe2+) glasses formed by the quenching of molten material ejected from Earth’s surface as a result of a hypervelocity impact. The vast majority of tektites are usually homogeneous glasses, but rare samples containing mineral inclusions can provide insights about the source material, sample thermal history, and tektite formation process. Tektites from two distinct strewn fields presenting Ca-phosphate inclusions detected from anomalous magnetic susceptibility were studied: one sample from the Ivory Coast tektite (ICT) field ejected at 1.07 Ma from the Bosumtwi crater (10.5 km in size) in Ghana and two Muong Nong type samples from the Australasian tektite field (MN-AAT) ejected at 0.79 Ma from a crater possibly situated in southeast Asia. In ICT, Ca-phosphate inclusions are systematically embedded in lechatelierite (SiO2 glass). In MN-AAT Ca-phosphate are either embedded in lechatelierite or in Fe-rich glass forming schlieren. Multiscale petrographic characterization using correlative microscopy associating scanning electron microscopy, microprobe and, transmission electron microscopy reveals that rounded inclusions in ivoirite are composed of acicular Ca-phosphates (merrillite) embedded in an amorphous P-rich glass. In MN-AAT, inclusions consist mostly of single droplets of Fe-Mg rich Ca-phosphate (structurally related to apatite), but few droplets often forming an emulsion texture show a complex assemblage of apatite, magnetite, pyroxene, and spinel growing from a Pt-rich nucleus. Diffusion profile around lechatelierite domains reveals maximum temperatures greater than 2200–2400 °C in the impact plume of the Australasian tektite and the Ivory coast tektite. Heating time is of the order of seconds-tens of seconds rather than minutes as previously suggested (20 s for MN-AAT and 5 s for ICT). The number, the density, and the fact that inclusions are entirely crystallized in MN-AAT support relatively slow cooling rates (<200 °C/h), in comparison with the faster cooling rates (>2000 °C/h) indicated by the precipitation of amorphous P-rich glass in ICT. In both impact events, ejecta that had been heated to high temperatures did not remain in the vapor plume for an extended period of time and landed rapidly (within tens of seconds) at a relatively high temperature (>1000 °C) on the Earth’s surface.Phosphate inclusions systematically embedded in lechatelierite in ICT provide clues about the source material. It suggests that the parent material for these silica-rich inclusions is not conventional detrital quartz. Rather, parts of lechatelierite domains may be inherited from a biogenic source that could be consistent with tropical soil (source of the phosphor) and its biomass (silica of plant origin). The reduction process that tektites record during their formation may be explained by superficial material since forests can contain a sizable mass of carbon that can reduce iron in tektites and produce platinoid-rich metallic nuclei and the Fe3+/ΣFe gradient recorded by the dendritic spinels.

The Application of Light-Assisted Drying to the Thermal Stabilization of Nucleic Acid Nanoparticles.

Background: Cold-chain storage can be challenging and expensive for the transportation and storage of biologics, especially in low-resource settings. Nucleic acid nanoparticles (NANPs) are an example of new biological products that require refrigerated storage. Light-assisted drying (LAD) is a new processing technique to prepare biologics for anhydrous storage in a trehalose amorphous solid matrix at ambient temperatures. In this study, LAD was used to thermally stabilize four types of NANPs with differing structures and melting temperatures. Methods: Small volume samples (10 μL) containing NANPs were irradiated with a 1064 nm laser to speed the evaporation of water and create an amorphous trehalose preservation matrix. Samples were then stored for 1 month at 4°C or 20°C. A FLIR C655 mid-IR camera was used to record the temperature of samples during processing. The trehalose matrix was characterized using polarized light imaging (PLI) to determine if crystallization occurred during processing or storage. Damage to LAD-processed NANPs was assessed after processing and storage using gel electrophoresis. Results: Based on the end moisture content (EMC) as a function time and the thermal histories of samples, a LAD processing time of 30 min is sufficient to achieve low EMCs for the 10 μL samples used in this study. PLI demonstrates that the trehalose matrix was resistant to crystallization during processing and after storage at 4°C and at room temperature. The native-polyacrylamide gel electrophoresis results for DNA cubes, RNA cubes, and RNA rings indicate that the main structures of these NANPs were not damaged significantly after LAD processing and being stored at 4°C or at room temperature for 1 month. Conclusions: These preliminary studies indicate that LAD processing can stabilize NANPs for dry-state storage at room temperature, providing an alternative to refrigerated storage for these nanomedicine products.

Controlling the volume fraction of glass-forming colloidal suspensions using thermosensitive host "mesogels".

The key parameter controlling the glass transition of colloidal suspensions is φ, the fraction of the sample volume occupied by the particles. Unfortunately, changing φ by varying an external parameter, e.g., temperature T as in molecular glass formers, is not possible, unless one uses thermosensitive colloidal particles, such as the popular poly(N-isopropylacrylamide) (PNiPAM) microgels. These, however, have several drawbacks, including high deformability, osmotic deswelling, and interpenetration, which complicate their use as a model system to study the colloidal glass transition. Here, we propose a new system consisting of a colloidal suspension of non-deformable spherical silica nanoparticles, in which PNiPAM hydrogel spheres of ∼100-200μm size are suspended. These non-colloidal "mesogels" allow for controlling the sample volume effectively available to the silica nanoparticles and hence their φ, thanks to the T-induced change in mesogels' volume. Using optical microscopy, we first show that the mesogels retain their ability to change size with T when suspended in Ludox suspensions, similarly as in water. We then show that their size is independent of the sample thermal history such that a well-defined, reversible relationship between T and φ may be established. Finally, we use space-resolved dynamic light scattering to demonstrate that, upon varying T, our system exhibits a broad range of dynamical behaviors across the glass transition and beyond, comparable with those exhibited by a series of distinct silica nanoparticle suspensions of various φ.

Assessment of activation energy of enthalpy relaxation in sucrose-water system: effects of DSC cycle type and sample thermal history

The purpose of this study is to critically analyze different methods of calculation of activation energy of relaxation in sucrose-water system from differential scanning calorimetry data. We consider the use of different thermal cycles for calculations together with Moynihan and Kissinger equations. We study the effect of two methods of glass transition temperature determination (half-step and inflection point) on the activation energy values. Along with experimental DSC data, we use the data simulated using Tool–Narayanaswamy–Moynihan model to validate the use of cooling and heating curves and to check the reproducibility of the activation energy calculations. The obtained results show that the thermal cycle with equal cooling and heating rates provides the most reliable data set and the glass transition temperature definition using inflection point rather than half step can be recommended for calculations. Moreover, due to technical reasons, heating rather than cooling scans provide the most reliable results of activation energy calculations. Furthermore, a simple method based on the width of the glass transition region shows reasonable results for single scan experiments. The activation energies of the glass transition in sucrose-water system with different water contents and different thermal histories were studied. Since it is impossible to apply traditional methods based on Moynihan equation for the activation energy evaluation for freeze-dried samples, we propose using another method based on the properties of the recovery peak. Combining the results obtained by different methods, we present a dependence of activation energy in sucrose-water system on water content. The results show that water decreases the activation energy of relaxation process in sucrose matrix.

Dodecagonal quasicrystals of oil-swollen ionic surfactant micelles

A delicate balance of noncovalent interactions directs the hierarchical self-assembly of molecular amphiphiles into spherical micelles that pack into three-dimensional periodic arrays, which mimic intermetallic crystals. Herein, we report the discovery that adding water to a mixture of an ionic surfactant and n-decane induces aperiodic ordering of oil-swollen spherical micelles into previously unrecognized, aqueous lyotropic dodecagonal quasicrystals (DDQCs), which exhibit local 12-fold rotational symmetry and no long-range translational order. The emergence of these DDQCs at the nexus of dynamically arrested micellar glasses and a periodic Frank-Kasper (FK) σ phase approximant sensitively depends on the mixing order of molecular constituents in the assembly process and on sample thermal history. Addition of n-decane to mixtures of surfactant and water instead leads only to periodic FK A15 and σ approximants with no evidence for aperiodic order, while extended ambient temperature annealing of the DDQC also reveals its transformation into a σ phase. Thus, these lyotropic DDQCs are long-lived metastable morphologies, which nucleate and grow from a stochastic distribution of micelle sizes formed by abrupt segregation of varied amounts of oil into surfactant micelles on hydration. These findings indicate that molecular building block complexity is not a prerequisite for the formation of aperiodic supramolecular order, while also establishing the generic nature of quasicrystalline states across metal alloys and self-assembled micellar materials.

Hydride growth mechanism in zircaloy-4: Investigation of the partitioning of alloying elements

The long-term safety of water-based nuclear reactors relies in part on the reliability of zirconium-based nuclear fuel cladding. Yet the progressive ingress of hydrogen during service makes zirconium alloys subject to delayed hydride cracking. Here, we use a combination of electron back-scattered diffraction and atom probe tomography to investigate specific microstructural features from the as-received sample and in the blocky-α microstructure, before and after electrochemical charging with hydrogen/deuterium followed by a low temperature heat treatment at 400 °C for 5 h followed by furnace cooling at a rate of 0.5 °C/min. Specimens for atom probe were prepared at cryogenic temperature to avoid the formation of spurious hydrides. We report on the compositional evolution of grains and grain boundaries over the course of the sample's thermal history, as well as the ways the growth of the hydrides modifies locally the composition and the structure of the alloy. We observe a significant amount of deuterium left in the matrix, even after the slow cooling and growth of the hydrides. Stacking faults form ahead of the growth front and the segregation of Sn at the hydride/matrix interface and on these faults. We propose that this segregation may facilitate further growth of the hydride. Our systematic investigation enables us discuss how the solute distribution affects the evolution of the alloy's properties during its service lifetime.

Thermochronology of the Angara–Vitim Granitoid Batholith, Transbaikalia, Russia

We reconstructed the history of the Angara–Vitim batholith (AVB), characterized by the formation of huge volumes of granitic magma using thermochronological analysis (40Ar/39Ar, U/Pb) and math testing selected age milestones on the basis of the model crystallization differentiation and dynamics of heat and mass transfer in a magmatic chamber. The consolidation time of the granite melt is estimated to be in the range of 320–290 Ma. Events with age are distinguished based on the presence of isotope dating clusters T1 = 245 ± 1 Ma; T2 = 212 ± 1 Ma; T3 = 156 ± 1 Ma; T4 = 125 ± 2 Ma. The thermal history of samples corresponding to the modern erosion level immediately imposes strict restrictions on the history of consolidation and cooling for rocks corresponding to the deep (>20 km) levels of the magmatic chamber. Calculations show that the lifetime of the residual melt at the deep levels of the magmatic chamber of the AVB can reach 100 Ma. Events with age 245 ± 1, 212 ± 1, 156 ± 1, 125 ± 2 Ma reflect the gradual transformation of the “semi-frozen granite layer” and the discrete nature of its tectonic exposure to the upper level of the Earth’s crust under the influence of tangential elastic deformations caused by the pulsating manifestation of intraplate mantle magmatism within the Siberian platform and its folded frame. The final stage of AVB tectonic exposure to the Earth’s surface (as a solid body) occurred from 60 Ma to the present time, reflecting the process of origin and development of the Baikal rift system.

Anomalous self-diffusion, structural and energy relaxations and temporal scaling laws in pure tantalum and pure vanadium metallic glasses

While most studies have considered diffusion in metallic glasses (MGs) to be normal, with a temporally asymptotic diffusivity at a given temperature (T), we report that the diffusion is anomalous and should be described as subdiffusion in pure Ta and pure V MGs—two examples chosen here because of chemical simplicity, fast relaxation, and minimal ambiguity. The diffusivity at a constant T (below the glass transition temperature) drops continuously with time t according to a negative power law instead of the commonly assumed exponential decay. To understand this, we trace the dynamic evolution of potential energy (Ep) and icosahedral short-range ordering (fico) of the system. We find that at large t, fico increases linearly with ln(t), causing Ep to decrease linearly with ln(t), with a remarkably simple linear correlation between fico and Ep. Based on Ep, we infer a continuously increasing effective migration energy barrier, also scaling linearly with ln(t), which perfectly recovers the negative power law time-dependence of diffusivity. The finding of anomalous diffusion and its origin in the fast-relaxing MGs calls out the need to distinguish potentially anomalous diffusion from normal diffusion in MGs, which is critical in disentangling time and temperature in experimental data and developing a robust theory for diffusion in MGs. The temporal scaling laws for diffusivity and structural and energy relaxations reported here may find their validity in other MGs and aid future theory development. In addition, we also discuss the effect of sample thermal history on the time-dependent diffusivity and how to prevent confusion caused by such effect.

Ultrasound and ethanol pre-treatments to improve convective drying: Drying, rehydration and carotenoid content of pumpkin

Among different effects, both ultrasound and ethanol technologies change the product structure and promote mass transfer when used as pre-treatments to improve the food drying. For the first time, their combined application was evaluated as pre-treatment to improve convective drying, as well as the properties of the dried food. Pumpkin cylinders were used as model food, being dried using air at 50°C and 0.8±0.1ms−1. Effects on drying and rehydration kinetics, as well as on the energy consumption and carotenoid preservation, were studied. Compared to the Control, all pre-treatments decreased the drying time in more than 48%. The combination of ethanol and ultrasound presented the greatest reduction in both drying time (59%) and energy consumption (44%). The pre-treatments also enhanced the rehydration properties in more than 28%: higher rehydration rate and an increase of water retention were achieved. Possible mechanisms, involving structure modification and mass transfer during pre-treatment and drying process were discussed. Additionally, the extraction of carotenoids due to pre-treatments was negligible when compared to the remarkable effect in avoiding this nutrient degradation during drying. Pre-treated samples preserved ∼100% of the carotenoid content, while the Control samples presented partial degradation (23%). This was explained by the negative effects evidenced by sample thermal history during drying. The results open new perspectives about an innovative method to improve the drying process and product quality by combining ethanol and ultrasound.

Non-equilibrium strain relaxation noise in the relaxor ferroelectric (PbMg1/3Nb2/3O3)1-x(PbTiO3)x

Large low-frequency polarization noise is found in some perovskite relaxor ferroelectrics when they are partially polarized, regardless of whether the polarization is accompanied by an applied electric field. The noise appears both in the ferroelectric and relaxor states, including the nominally ergodic paraelectric state at temperatures above the susceptibility peak. Since it is present whenever the samples have non-zero average piezoelectric coefficients, but not otherwise evident, it appears to be a response to mechanical strain changes. Dependence of the noise on sample thermal history indicates that non-equilibrium strain relaxation is the source, even in the temperature range for which the sample is nominally ergodic. Non-equilibrium noise in the absence of net piezoelectricity is found at somewhat higher frequencies. Related materials lacking a metastable non-equilibrium cubic bulk phase and a symmetry-broken surface layer show very little of the anomalous low-frequency noise. The implications for a non-equilibrium origin of the skin effect are discussed.

Thermally induced cation redistribution in fluor-elbaite and Fe-bearing tourmalines

An Fe-rich fluor-elbaite was thermally treated in air and hydrogen atmosphere up to 800 °C to study potential changes in Fe- and Al-ordering over the octahedrally coordinated Y and Z sites. Overall, the experimental data (structural refinement, electron and ion microprobe, Mossbauer, infrared and optical absorption spectroscopy) show that thermal treatment of fluor-elbaite results in an increase of Fe contents at the Z site balanced by an increase of Al at the Y site. On the basis of this and previous experimental studies on Fe–Mg–Al-bearing tourmalines, it can be stated that the intersite Fe–Mg–Al exchange rates are significant at temperatures around 700–800 °C. Thermal treatment results in an increase of ca. 0.30 Fe atoms per formula unit at the Z site compensated by a similar increase of (Mg + Al) at the Y site, following the exchange reaction YFe + Z(Mg + Al) → ZFe + Y(Mg + Al). Since the tourmaline nomenclature is based on the occupancy of ions at each structural site, the intersite Fe–Mg–Al ordering may determine the tourmaline species. This means that effectively the name associated with a given composition may be a function of the sample thermal history.

Zircon grain selection reveals (de)coupled metamictization, radiation damage, and He diffusivity

Broad variability in intrasample effective U concentration (eU) in zircon crystals is required to characterize zircon (U-Th)/He (zircon He) date-eU relationships that resolve thermal histories in ancient rocks. We describe a new protocol for selecting grains for zircon He thermochronometry that leverages visual crystal metamictization, a proxy for radiation damage accumulation, to generate a range in intrasample eU and zircon He date-eU/metamictization trends. Plane polarized light images, eU concentration, and zircon He data from 59 individual grains from six Proterozoic and three Archean North American granitoids reveal a progressive increase in crystal opacity and discoloration that correspond to increasing eU in each sample. Zircon eU ranges from 89 to 1885 ppm across the dataset and individual samples have eU variations of 538 ppm to 1579 ppm. Three Proterozoic samples yield distinct negative zircon He date-eU patterns with individual dates of ~908 to 20 Ma. Archean grains collectively define a negative date-eU trend from ~661 to 4 Ma. In contrast, three other Proterozoic samples yield uniform ~25 Ma zircon He dates over an 1800 ppm eU range, despite increasing eU with increasing metamictization. Simple thermal history models that account for coevolution of zircon radiation damage and He diffusion demonstrate visible metamictization and diffusivity are either coupled or decoupled depending on a sample's thermal history. Zircon He date-eU/metamictization pediments develop when variable but high eU grains with variable visual damage and closure temperature close with respect to He at the same time. Uniform dates with progressive increases in eU and visual damage, reflect decoupled He diffusion and visual damage, are associated with rapid cooling, and/or imply some forms of damage in zircon may not fully anneal.

Thermal hysteresis of electromechanical characteristics of Y + 42° cut LiTaO3 single crystals

The temperature stability of electromechanical characteristics of Y + 42° cut LiTaO3 crystals has been studied using dielectric spectroscopy and static and dynamic piezoelectric effect measurements. It has been shown that, during thermal cycling, the piezoelectric properties of crystalline LiTaO3 samples exhibit a thermal hysteresis and that the thermal history of samples influences their resonance and antiresonance frequencies.

Comparison of Structural Relaxation Behavior in As-Cast and Pre-Annealed Zr-Based Bulk Metallic Glasses Just below Glass Transition

The i-relaxation of pre-annealed Zr55Cu30Ni5Al10 bulk metallic glasses (BMGs) was compared with that of as-cast Zr-based BMGs including Zr55Cu30Ni5Al10. The i-relaxation was investigated by volume relaxation. The relaxation behavior was well described by a stretched exponential relaxation function, � ðt Þ� exp½�ð t=¸iÞ ¢ i � , with the isothermal relaxation time, ¸i, and the Kohlrausch exponent, ¢i. The ¢i exhibited the strong temperature dependence for the pre-annealed BMG, while the weak temperature dependence was visualized for the as-cast BMG similar to the dynamic relaxation. The ¸i’s were modified by Moynihan and Narayanaswamy-Tool-Moynihan methods that reduce the difference in the thermal history of sample. As a result, the relaxation kinetics in the glass resembled that of a liquid deduced from the behavior of viscosity in the supercooled liquid. [doi:10.2320/matertrans.M2015023]

Primary Chemical Sequence Ultimately Determines Crystal Thickness in Segmented All-Aliphatic Copolymers

Thermoplastic elastomers (TPEs) are known to exhibit a phase-separated morphology which depends on their chemical structure and processing. The design of novel TPEs with predefined properties which are also independent of the material thermal history has so far remained a challenge. The focus of this work is on the semicrystalline morphology of all-aliphatic thermoplastic elastomers consisting of alternating polytetrahydrofuran (PTHF) segments and uniform glycine or β-alanine bisoxalamide units. The thickness of the hard segment crystals was found to be highly monodisperse and independent of the sample thermal history. Using Nanocalorimetry, we observed that at cooling rates as high as 12 000 °C s–1 the bisoxalamide segments can still crystallize although the crystallization temperature decreases by ca. 26 °C. The surface free energy of the hard block crystals is found to be extremely low (∼18 mJ·m−2), which is likely due to the entropic contribution of soft segments forming tie chains bridging the neighboring crystals. To investigate the combined effect of crystal orientation and phase transitions, simultaneous time-resolved X-ray scattering and mechanical tensile tests were performed. Upon stretching, elastomeric PTHF segments with lengths above 1000 g mol–1 crystallize at ambient temperatures. Under these conditions two main morphologies were observed: at low strains the long axes of the fibril-like crystals were oriented parallel to the flow direction, whereas higher strains caused bisoxalamide crystal fragmentation and changed their preferential direction to the one perpendicular to the drawing direction. The chain tilts in the bisoxalamide crystals were calculated from the characteristic four-spot SAXS patterns and were ∼5°–16° in the case of glycine end-groups and 24° for alanine and propyl terminal groups. To our knowledge, this is the first attempt to determine the chain tilt for the nonlamellar crystals in block copolymers.

Phase behavior of Safaniya vacuum residue

The phase behavior of Safaniya vacuum residue (Saudi Arabia) from (200 to 520) K at atmospheric pressure was investigated and the phase states were determined from nanofiltered samples with pentane-asphaltene mass fractions wA between 0.048 and 0.555 based on joint treatment of heat capacity data from differential scanning calorimetry and phase angle data from rheological measurements. SARA and elemental composition data of the oil are also considered and some assumptions on more detailed oil chemistry have been made based on a predictive heat capacity correlation. A pseudo binary phase diagram is constructed based on the behavior of retentate free permeate and permeate free retentate. Compositionally, these materials comprise pentane maltenes and asphaltenes respectively to a first approximation. These pseudo-components, partitioned at 473K, are shown to behave independently. A broad phase transition arising in Safaniya vacuum residue phase behavior is shown to comprise overlapping but independent transitions in these two pseudo components. The temperature range of the combined glass+melting transition depends on sample thermal history (both long-term and short-term). Based on this representation, Safaniya vacuum residue comprises four phases drawn from crystalline and liquid maltene-rich phases as well as crystalline and liquid asphaltene-rich phases from 265K to 360–370K. It is also shown that the phase behavior of chemically separated pentane asphaltenes differs significantly from that of the nanofiltered asphaltene-rich material. The phase diagram of Safaniya vacuum residue is compared with that of Maya crude oil [M. Fulem, et al. Fluid Phase Equilib. 272 (2008) 32–41] and Athabasca bitumen [A. Bazyleva, et al. J. Chem. Eng. Data 56 (2011) 3242–3253]. The differences in the phase behavior of these oils are traced to differences in their composition.

Influence of thermal history on relaxation process in barium titanate ceramics

The mechanical and dielectric properties of barium titanate ceramics with different thermal histories (such as cooling, heating and aging) were investigated in temperature range −10°C to 140°C at various frequencies. Experimental results clearly showed that in the tetragonal phase, the relaxation processes displayed completely different behaviors depending on the sample's thermal history. A similarity of the thermal history effect on the mechanical and dielectric relaxation behavior was observed, where the oxygen vacancy symmetry and the domain wall mobility played an important role. It was proposed that dielectric relaxation of the aged sample was associated with the reorientation of the dipoles due to the relaxation of oxygen vacancies at A and B sites. The obtained results may provide a helpful guidance for improving the dielectric performance of ferroelectric aged capacitors after a long working time.

Structural features and functional properties of water in model DMPC membranes: thermally stimulated depolarization currents (TSDCs) and Fourier transform infrared (FTIR) studies

Thermally stimulated depolarization currents (TSDCs) and Fourier transform infrared (FTIR) spectroscopies were employed to investigate the state of water incorporated in a model DMPC (dimyristoyl-phosphatidylcholine) membrane. The lipid multilayers, highly inhomogeneous from the dielectric point of view, originate complex TSDC spectra critically dependent on the sample water content and thermal history. Different temperature ranges were chosen to polarize the sample, i.e. 100–300 K (type I) and 100–285 K (type II). The purpose of the latter choice was to avoid any sample heating above the DMPC phase transition temperature (295 K) along the sample polarization. According to the results, water in a fully hydrated system (aw = 0.92) (1) is ordered around the hydrophilic head molecular groups, (2) is layered in the interbilayer space and (3) penetrates among the hydrocarbon chains. It can assume different local structural configurations depending on the lipid packing. Irreversible conformational transitions in the lipid array system were monitored as a consequence of different dehydration treatments.FTIR absorption measurements were performed to study the water sorption kinetics into a DMPC thin film. The water related OH band was decomposed into three components, describing three water states, with different propensity to the H-bond formation. The changes of the lipid characteristic groups (CH2/CH3, and C=O) absorption bands as a function of increasing hydration level were monitored and discussed.

Further investigation of the initial fission-track length and geometry factor in apatite fission-track thermochronology

The external detector method (EDM) is a widely used technique in fission track thermochronology (FTT) in which two different minerals are concomitantly employed: spontaneous tracks are observed in apatite and induced ones in the muscovite external detector. They show intrinsic differences in detection and etching properties that should be taken into account. In this work, new geometry factor values, g, in apatite, were obtained by directly measuring the ρ ed /ρ is ratios and independently determined [GQR] ed/is values through the measurement of projected lengths. Five mounts, two of which were large area prismatic sections and three samples composed of random-orientation pieces have been used to determine the g-values. A side effect of applying EDM is that the value of the initial confined induced fission track, L 0 , is not measured in routine analyses. The L 0 -value is an important parameter to quantify with good confidence the degree of annealing of the spontaneous fission tracks in unknown-age samples, and is essential for accurate thermal history modeling. The impact of using arbitrary L 0 -values on the inference of sample thermal history is investigated and discussed. The measurement of the L 0 -value for each sample to be dated using an extra irradiated apatite mount is proposed. This extra mount can be also used for determining the g value as an extension of the ρ ed /ρ is ratio method. Eight apatite samples from crystalline basement, with grains at random orientation, were used to determine the g-values. The results found are statistically in agreement with the values found for apatite samples (from Durango, Mexico) measured in prismatic section and also measured at random orientation. There was no observable variation in efficiency regarding crystal orientation, showing that it is relatively safe using non-prismatic grains, especially in samples with paucity of grains, as it is the case of most basin samples. Implications for the ζ-calibration and for the calibration of the direct (spectrometer-based) fission-track dating are also discussed.

Experimental assessment of nonergodicity in tetracene single crystals

Dielectric spectroscopy is applied to the study of phase transitions in the range from room temperature down to 12 K in single crystals of tetracene (TEN), a well-known material which, like most organic semiconductors, shows polymorphism. The present analysis, through proper experimental procedures, permits us to identify the wide range ($T$ &gt; 150 K) of instabilities and of strong hysteresis effects as clear evidence of a nonergodic state and then also to clarify some open questions, first of all the role of the sample thermal history, including the temperature variation rate and some apparent sample dependence of the results. Furthermore evidence is obtained of two transitions at 144 and 45 K.